In the most common form of scanning monochromator (used for example in spectrometers for the analysis of electromagnetic radiation), when the dispersing element is a concave spherical grating the instrument construction and grating thereof are based on Rowland's principles. The grating is a part of a sphere and has lines ruled across its concave surface which, when projecting onto a plane surface, are straight, parallel and equidistant. The entrance slit or source, the grating, and the detector (or exit slit, photographic plate or observing telescope) are positioned around the circumference of a circle whose diameter is equal to the radius of curvature of the grating. The grating is relatively short and arranged tangentially to the said circle, called hereinafter the Rowland circle with the ruled lines at right angles to the plane of the circle. Such instruments have broad band properties but the imaging is not perfect, even in the meridian plane for small gratings, and it deteriorates rapidly as the grating is extended in length to increase its light grasp (or angular aperture). The aberrations lower spectral resolution and intensity; thus relatively long detection times are necessary.
The greatest care has to be taken in the ruling of the lines on the grating. The performance of the grating is dependent upon the limitations of the ruling engine. Irregularities (random errors) and systematic variation in the spacing across the grating surface are considered to constitute serious faults. B. Gale states in his article entitled "The Theory of Variable Spacing Gratings" (Optica Acta 1966 Vol. 13 No. 1, pages 41-54) that although a variable spacing of the lines on a plane grating may provide improved focussing properties, in the case of a concave spherical grating no such improvement can be obtained by any choice of ruling pattern. We have made the surprising discovery however that a choice of spacing of the lines on a spherical or toroidal grating will provide better imaging if the focussing geometry in which it is used is modified from the given by Rowland's principles.
Accordingly it is an object of the present invention to provide a new and improved scanning monochromator, which may for example be used as part of a spectrometer. It is a further object of the invention to provide a scanning monochromator having a spherical or toroidal reflecting grating in which the spacing of the lines on the grating is varied, being chosen to provide improved and enhanced imaging of the source.